The present invention relates to ink follower, which follows water-base ink filled in an ink reservoir of a ballpoint pen, and also to a method for manufacturing the same.
The ink for a water-base ballpoint pen has a viscosity of as low as 50 mPa sec to 3 Pa sec, while the ink for an oil-base ballpoint pen, though it has a similar structure to a water-base one, has a viscosity of 3 to 20 Pa sec. Consequently, the ink filled in a water-base ballpoint pen may leak out when the pen is left upward or sideways. Moreover, even a small impact made on the pen may cause its ink to scatter and to stain the hand or the clothes.
There have been conventional arts for a water-base ballpoint pen with its ink in its ink reservoir that it is equipped with an ink follower composed of a gelled material, or a mixture of the gelled material and solid material. The aims of the arts are to make the ink follower follow the ink smoothly, to make the pen endure an impact when dropped, to prevent the ink from back flow, to give the pen a good appearance, and so on. A common feature of such arts is that the ink follower, which has pseudo-plasticity, is made from hardly-volatile or non-volatile solvent which is supplemented with some kind of thickener in order that the ink follower may not flow backward when the pen is left sideways or upward.
Moreover, conventional ink follower for water-base ballpoint pens often has very low viscosity and tenacity, as compared with that for conventional oil-base ballpoint pens, which often has equal viscosity to common grease used for lubricant.
About as much as 50 to 300 mg of ink is required for writing a line of 100 m in length by a water-base ballpoint pen holding the ink in the ink reservoir, while only 10 to 30 mg of ink is required by an oil-base ballpoint pen. Thus, the ink follower for the water-base ballpoint pens is required to have a strict ink-following performance, and is, therefore, mainly of low viscosity.
Lubricant grease with low viscosity and consistency generally has such low stability that oily matter likely separates when left to stand. In addition, the high mobility of thickener in the lubricant grease is likely to cause the grease to lose homogeneity easily by forming a mixture of coarse and dense portions.
The lower the viscosity of the thickener of the grease is, the less effectively the thickener is dispersed by a disperser such as a double-roll mill, a triple-roll mill, a kneader or a planetary mixer, any one of which is suitable for substances with high viscosity. The thickener is, however, not so low in viscosity as to be capable of being mixed effectively by a disperser such as a bead mill, a sand mill or a homogenizer, any one of which is suitable for substances with low viscosity. Inefficient dispersion causes not only time-dependent instability but also lot-to-lot instability in viscosity and uniformity.
The ink follower for the water-base ballpoint pens consists of materials similar to the lubricant grease, and exhibits time-dependent behaviors based on similar physical laws.
However, if the oily matter separates from ink follower, it affects writing adversely by reacting with surfactant in the ink, or by forming oil droplets which block the ink passage.
Moreover, ink follower lacking homogeneity is separated into a portion following the ink and portions adhering to the inner wall of the ink reservoir. The adhering portions not only give the pen an unpleasant appearance, but also mean a corresponding loss of the ink follower, resulting eventually in its failure to perform its function of, for example, preventing the ink from volatilizing or from leaking.
The lubricant grease and the known ink follower have a common defect, too.
When they are used as ink follower in a water-base ballpoint pen which holds the ink in a cylindrical or similarly shaped ink reservoir with an inside diameter of 2.5 mm or larger, bubbles often occur between the ink and the ink follower by the passage of time. Moreover, bubbles or cracks, which have not seen initially, often occur in the ink follower (or the lubricant grease used as a substitute therefor). Since ink follower is nearly liquid rather than semi-solid, it is very questionable whether an expression of xe2x80x9ccracksxe2x80x9d is appropriate, but the greasy matter is cracked in appearance. We, the inventors of the present invention, call these phenomena xe2x80x9cbubblingxe2x80x9d. Once the bubbles occur between the ink and the ink follower, it grows larger and interrupts the contact between the ink and the ink follower. Then, the ink follower is urged by the vapor pressure of the ink toward the tail end of the pen, and eventually falls off. The ink follower having cracks or the like loses its function of keeping the ink from contact with the air.
These phenomena are presumably due to the invisibly fine bubbles that may exist in the ink follower or lubricant grease when manufactured. The bubbles gather with the passage of time, and tend to escape from the pen.
The bubbling is a serious defect in this kind of water-base ballpoint pen.
Commercially available ballpoint pens are subjected to a strong centrifugal force for debubbling. Debubbling by a strong centrifugal force is, however, not always effective for removing invisibly fine bubbles, but can only reduce the percentage of bubbling to about ⅕ to {fraction (1/20)}.
Moreover, the centrifuging is not a suitable method for debubbling for pens with a pigment ink, particularly the ink containing a pigment with a true specific gravity of 4 or higher, since a strong centrifugal force promotes the sedimentation of the pigment.
Thus, in order to prevent the contamination of fine bubbles in advance, conventional centrifugal debubbling when the pens are assembled may be carried out strongly. However, too strong centrifugal debubbling causes defects such as deformation of pen point or the connecting part of the ballpoint pen. Further, it may sometimes happen that the pigment in the ink is sedimented and clogs at the pen point resulting in poor writing and that is significant when pigment of a high specific gravity such as titanium oxide and metal powder is contained in the ink.
The objects of this invention are to dissolve the defect that conventional ink follower for a water-base ballpoint pen has lot-to-lot and time-dependent instability of quality, and to provide ink follower which has time-dependent stable performance for mass-production and a method for manufacturing thereof.
The other object of the present invention is to provide a method for manufacturing the ink follower by which the time-dependent growth of bubbles, which is a defect of conventional ink follower for water-base ballpoint pens, is dissolved, without any above-mentioned bad influence upon the ballpoint pens caused by a strong centrifugal force after filling the ink and the ink follower into a ballpoint pen holder.
As a result of our diligent study of above problems, we have found that, by homogenizing particulate silica, clay thickener, metal soap, or organic thickener microscopically highly, the thickener constantly exhibits its best performance. And we have also found that the ink follower, therefore, showed more time-dependent stability and less lot-to-lot instability. Thus we have completed our present invention.
Moreover, as a result of diligent study of above problems, we found that conventional centrifugal debubbling during the assembly of ballpoint pens, which is said to be an essential process, is not so strong when centrifugal debubbling upon ink follower of water-base ballpoint pens is previously performed. Upon this knowledge, we have found some conditions that the invisible fine bubbles contained in particulate silica, clay thickener, metal soap and organic thickener can be highly removed microscopically without any bad influences on writing property of the pens, and thus we have accomplished the present invention.
Lubricant grease and ink follower for a water-base ballpoint pen are prepared from similar materials by similar processes, but are clearly different from each other from a technical standpoint.
The lubricant grease is usually used for lubricating, and is, therefore, made to have high structural viscosity and yield value lest the oily constituent of the grease drip from a point where the grease is applied. On the other hand, the age ink follower for a water-base ballpoint pen is held in a reservoir with no opening except its rear end, and is used in an environment in which there is no sliding matter except itself. Therefore, the structural viscosity and yield value of the ink follower may be low. It would rather be correct to say that it is necessary for the ink follower to be low in structural viscosity and yield value in order to follow the ink smoothly.
Fine particulate powder such as inorganic thickener (particulate silica, alumina or titanium dioxide), inorganic or organic pigment and fine resin particulate, which gains structural viscosity in liquid, generally shows a lower thickening effect and a lower yield value when it is well-dispersed.
Clay thickener and organic thickener, which exhibit thickening effect by swelling with a solvent, tend to show a lower yield value when they are well-dispersed in liquid. So does metal soap.
Although the thickener of the ink follower, such as particulate thickener and clay thickener, appears to be thoroughly wet with the solvent, microscopically small bubbles exist in the core of the particle of the thickener because of its thickening effect that prevents the solvent from permeating thoroughly to its core. This is evident from the fact that the grease or, the ink follower, though it appears bubble-free, produces a large number of bubbles under reduced pressure at much lower temperature than the boiling point of its oily constituent. So does metal soap thickener that seems to have, being prepared at high temperature, an advantage in permeability of the oily constituent.
In the present invention, we have improved the wetting of every particle of the thickener with solvent and caused the thickener to always exhibit its ability to its maximum degree. Thus we have given the ink follower stability in lot-to-lot viscoelasticity and performance. Moreover, the thickener being homogenized, we have successfully obtained ink follower for a water-base ballpoint pen with very good time-dependent stability. This invention relates to a method of manufacturing such follower under the conditions that satisfy above requirements.
The solvent used as the base oil for the ink follower is selected from polybutenes with a molecular weight of 500 to 3000, liquid paraffin, mineral oil such as spindle oil, silicone oil and so on. They do not dissolve in a water-base ink, and have only a small volatile loss. They generally have a better wetting property with resins, such as polypropylene, polyethylene and so on, used for an ink reservoir than that of water-base ink. Thus the consumption of the ink is easy to recognize.
Polybutenes and silicone oils, though some kinds of them are highly volatile, can withstand for two years or more at room temperature if their volatile loss is not more than about 0.2% by weight under a JIS C-2320 method at 98xc2x0 C. for five hours.
The volatility of polybutenes largely depends upon their molecular weight. Polybutenes with average molecular weight more than about 500 may satisfy the above volatile loss.
Since the volatile loss of silicone oils also depends upon their molecular structure, it cannot be determined only by their molecular weight. Therefore, the volatile loss of silicone oils is recommended to be measured practically by the method as described above.
The thickener used for the present invention is preferably hydrophobic or insoluble. Hydrophilic thickener sometimes migrates into the ink through the surface between them. As a result, the ink follower loses of its viscosity, and the ink suffers an ill effect of being unable to write. However, hydrophilic thickener can be used if appropriate measures such as, for example, water-repelling treatment made to the thickener or the ink follower and the ink composition hard to be affected by the thickener are taken.
Preferred examples of the thickener are:
particulate silica with methylated surfaces such as Aerozyl R-972, R-974D, R-976D and RY-200 (Nippon Aerozyl Co., Ltd.),
organic thickener such as Leopar KE (Chiba Powder Manufacturing Co., Ltd.),
clay thickener, which has hydrophobized surfaces by onium treatment, such as dimethyldioctadecyl ammonium bentonite, and
insoluble metal soap such as lithium stearate, aluminum stearate and sodium stearate.
Each of above substances may be used alone, or may be used in combination with others. The total amount of thickener is preferably from 1 to 10% by weight of the ink follower.
Hydrophilic thickener, such as Aerozyl #200, 380, 300, 100 and OX50 (Nippon Aerozyl Co., Ltd.), particulate alumina and ultra-particulate titanium dioxide, can be prevented from interfering with the ink when the ink follower contains the substances such as surfactant, silane coupling agent, fluorocarbon, and methylhydrogen silicone, each of which has a hydrophilic-lipophilic balance (HLB) value of less than 4, preferably of less than 2. When silicone oil is used the for base oil of the ink follower, it is often possible for the ink follower, without adding other substance, to avoid interfering with the ink.
It is effective to use an additive such as a surfactant to the ink follower in order to improve its property of following the ink. Even irrespective of the kind of surfactant, it is not preferable to use the surfactant that dissolves in the ink during storage, but is preferable to use nonionic surfactant with an HLB (hydrophile-lipophile balance) value less than 4. Moreover, the so-called fluorine-surfactant and silicone-surfactant are the most preferable additives for the present invention, in which the microscopic fine bubbles are eliminated by wetting the thickener thoroughly by pressure bubbling, since they can drastically lower the surface tension of the base oil.
It is also preferable for the object of this invention to add above-mentioned silane coupling agent, methylhydrogen silicone, etc. since they are effective for stabilization of the dispersion of the thickener, homogenization and hydrophobization. It is very preferable to use additives unless it makes an ill effect for the stability of the ink follower and for the quality of the ink.
The amount of these additives to be used is generally from 0.01%, which is minimal effective concentration, to about 5% by weight. The amount over 5% by weight does not produce any better result, though it may not present any problem in quality.
Since the present invention includes a method for manufacturing, it will be illustrated in detail by way of Examples. Conceptionally, it is based upon the idea that the invisible bubbles in the gelly substance or, more specifically, in the thickener are swollen under depressurization and eliminated from the system. It is also based upon the idea that the surface tension of the solvent is reduced by raising the temperature so that even the invisible fine bubbles are wetted.
It goes without saying that stirring results in better efficiency both under depressurization and under heating. Moreover, far better results are available when depressurization, heating and stirring are carried out at a time although it is better that the temperature is not so high when strongly depressurized at 0.1 atm or lower. A rough aim is around 60xc2x0 C.
Although ink follower containing fewer bubbles is prepared when a double-roll mill or a triple-roll mill is used at high temperature, it should be subjected to a final debubbling under depressurization by transferring it to a stirrer such as a kneader and a planetary mixer capable of depressurizing and heating.
Probable examples for the methods of debubbling are;
to eliminate the bubbles, which are swollen under reduced pressure, by spraying at least one kind of substance such as water, organic solvent such as alcohol and surfactant such as anti-bubbling agent; and
to mix bubble-breaking agent previously.
These methods may, however, leave the matter sprayed to the ink follower or the component irrelevant to the property of the ink follower. Therefore, debubbling by depressurization in which no additives are used is preferred.
Depressurization must be done under 0.2 atm or lower. The value of 0.2 atm is a result of experiences by the present inventors and we have no scientific explanation for that. Since we have intentionally used hardly-volatile solvent, we have not experienced the boiling even at a low pressure under the temperature less than or equal to 60xc2x0 C. The bubbles are not sufficiently eliminated at 0.2 atm or higher even in the state at high temperature in which the viscosity lowers.
The lower limit of the pressure should be low enough for the base oil not to boil. The present inventors are sure that lower pressure will cause a better result that the bubbles are well eliminated. However, any kind of hardly-volatile solvent will no longer exist in absolute vacuum. In addition, many of non-volatile and hardly-volatile solvents are in a state of a mixture of substances having different molecular weights and different positions of double bond and cyclic moiety. Therefore, some of them partly evaporate at high temperature and low pressure.
In the case of polybutene for example, what is usually called a molecular weight is an average and the polybutene is an aggregate of polybutenes having the molecular weights near the average value. When such polybutene is exposed at high temperature and low pressure, components having lower molecular weight are lost.
Quality control for each manufacture lot is apt to be dominated by the lot-to-lot difference of polybutene, but, since the components which are easily evaporated are firstly lost, time-dependent stability is rather good.
The present invention is also effective to a method where the substance dispersed keeping high viscosity is later diluted with solvent or the like. For example, while a triple-roll mill is used in the case of a compound in the Example 1 of the present invention, the same efficiency can be achieved without the use of the triple-roll mill according to a manufacturing method of the present invention in which the materials are well kneaded by a planetary mixer until the step for the triple-roll mill, added with mineral oil and stirred with heating at around 100xc2x0 C., and then adjusted to the temperature at 60xc2x0 C. or lower using cooling water or the like to debubble by depressurization. In order to disperse more efficiently, it is recommended to knead under depressurization prior to addition of the mineral oil or to knead after raising the temperature.
An example for a method of filling the ink follower of the present invention is that ink is filled in an ink reservoir, a pen point is attached and then ink follower is charged. After that, a strong centrifugal force is applied by means of a centrifugal separator in the direction from the tail end to the pen point whereby the ink follower is filled with a good appearance containing no air or the like between the ink and the ink follower.
In the case of the ink follower according to claims 19 to 21, a centrifugal force of 200xc2x7g (where g is a gravitational acceleration) is applied so that the ink follower is previously debubbled.
Regardless of water-base or oil-base ballpoint pens, the ballpoint pens where the ink is directly filled in an ink reservoir have conventionally applied a centrifugal force in order to debubble and to push the ink forward to the pen point. The object of this debubbling is to mechanically eliminate the bubbles existing between the pen point and the tail end of the ink reservoir, and the object is also to remove the visible large bubbles.
As compared with the centrifugal force applied to the pen point at that time, the value of 200xc2x7g seems to be too small even for the present inventors but it should be noted that the said centrifugal force is not that which is applied to the tip of the ballpoint pen but that which is applied to ink follower itself.
In the present invention, an object is to remove fine bubbles and, therefore, a strong centrifugal force is necessary. The above-mentioned conventional centrifugal force applied to a ballpoint pen in its manufacture is as strong as 1000xc2x7g or even more than 2000xc2x7g at the pen point, while it seldom reaches 200xc2x7g at the rear part of ballpoint pen holder where the ink follower is present since the radius from the center of the centrifugal separator is short.
Strong centrifugation has conventionally been applied to this kind of water-base ballpoint pens. The condition xe2x80x9cat 2800 rpm for 10 minutes using a centrifugal separator of type H-103N by Kokusan Enshinkixe2x80x9d mentioned in the conventional patent specifications is very strong as an assembling step for a ballpoint pen and the tip of the ballpoint pen is protected by a special tool working as a buffer. In the machine where the effective orbit radius of the tip of the pen when this buffer tool is used is 15 cm, the tail end of the ink follower is at the position of 2-2.5 cm from the center of the centrifugal separator and, as a result of calculation, a centrifugal force around 200xc2x7g is applied.
At that time, we the present inventors were worrying about the time-dependent contamination of bubbles in a water-base ballpoint pen and in the ink follower but, from various experiences, we have found its causes and the countermeasure for that, and have achieved the present invention.
From various experiences, we the present inventors have found that the minimum centrifugal force for removing the fine bubbles in the ink follower filled in the holder of the ballpoint pen is 200xc2x7g as a centrifugal force to the center of gravity of the ink follower while the sufficient centrifugal force is about 350xc2x7g. When 350xc2x7g is applied as a centrifugal force for removing the fine bubbles in the ink follower in the holder, the cracks or the like did not occur in the ink follower due to the bubbles. From our further experiences, it has been found that, when the centrifugal force exceeds 700xc2x7g, no bubble occurred even at the interface between the ink and the ink follower. However, such a centrifugal force is used too much, ill effects are resulted.
Needless to say, the present invention relates to a part of the art for ballpoint pens and, therefore, it is an object to manufacture ballpoint pens having a good property as a whole Therefore, in order to apply a strong centrifugal force to the ink follower located at the tail end area of the ballpoint pen, several-fold of the centrifugal force is imposed on the tip of the ballpoint pen and, as a result, the tip and the connecting part are physically deformed.
Further, when heavy pigment of the true specific gravity greater than 4 or pigment such as pearl or metal powder whose particle size is unable to make small is used, a strong centrifugal force should not be applied to the pen as a whole. Accordingly, it is important to make sure how much centrifugal force is applied to achieve this effect.
A strong centrifugal force is not to be applied upon assembling the ballpoint pen in order to prevent the deformation of the tip or the connecting part between the tip and the ink reservoir of the ballpoint pen or to prevent the sedimentation of the pigment in the ink.
An idea of the present invention is that a strong centrifugal force is applied to the ink follower itself whereby the fine bubbles are previously removed. In that case, it is suggested to apply a centrifugal force of 200xc2x7g or, preferably, 350xc2x7g or not less than 700xc2x7g.
However, that may cause separation of oil or unbalanced viscosity in the case of the ink follower whose fluidity is of big account. In that case, the centrifugal force applied to the ink follower should be as small as possible but it should be, as mentioned above, at least 200xc2x7g.
Both of the centrifugal force applied to centrifugally debubble the ink follower only and that applied to the ballpoint pen are fundamentally the same since both of them aim to eliminate the fine bubbles.
Namely, the said force is at least 200xc2x7g and, in order not to result in cracks in the ink follower, it is 350xc2x7g or, preferably, 700xc2x7g.
The difference from the case where debubbling is carried out after assembling the ballpoint pen is that, even a weak centrifugal force applied for a long time is effective.
The total centrifugal force when the time for application is taken into consideration is therefore the total energy applied thereto.
It may be said that sedimentation of the pigment of the ink is more strongly affected by such a total centrifugal force rather than by the strength of the temporary centrifugal force. That is simply considered as a problem of sedimentation since the pigment in the ink is almost wetted by its vehicle and that is a relation between the pigment (and the dispersing agent thereof) and other liquid components (and the dissolved thing therein) in the ink. The sedimented amount of the pigment is almost correlated to the product of time and gravitational acceleration.
On the other hand, the wetting of the fine particles or the clay thickener to the base oil of the ink follower is not so good as noted in the relation between the vehicle and the pigment in the ink whereby the bubbles are present. Accordingly, it is necessary to eliminate the bubbles adsorbed with the solid and the ones unable to come out due to physical hindrance. So that is not a simple problem of sedimentation. In order to release the adsorbed fine bubbles and also the bubbles due to a physical hindrance, an absolute power of force is necessary.
Therefore, in the case of conducting a centrifugal operation for the ink and for the ink follower at a time, it has been believed to be ideal to apply a strong centrifugal force in a short time.
When a centrifugal force is applied to the ink follower only, it is not necessary to think of the sedimentation of the pigment and, therefore, there is no necessity of too much anxiety about the total centrifugal force. When a centrifugal force is applied for a long time whereby the thickener is well wetted, then the fine bubbles are eliminated at a time.
However, the absolute power of the centrifugal force as mentioned above is still necessary. Even when too weak gravitational acceleration is applied for a long time, the fine bubbles are still adsorbed with the thickener or shut up therein due to a physical hindrance and are unable to be removed.
The turning point of the centrifugal force whether it is sufficient or insufficient is 200xc2x7g.
The reason why the minimum value which is effective in the case of assembling a ballpoint pen holder is also 200xc2x7g is that the fine bubbles adsorbed with the thickener or enclosed therein due to a physical hindrance are removed and, therefore, their physical behavior is presumed to be the same.
When a centrifugal force is applied to a ballpoint pen holder, the time for application is usually 5-10 minutes or shorter and at most 30 minutes. The above-mentioned finding is also based upon the test result carried out for the time within this range. When the time is longer than 30 minutes, poor writing probably due to sedimentation of the pigment may be noted that even in the case of pigment ink having a good dispersibility is used.
When only ink follower is centrifuged, the debubbling effect in a short time is the same as the case where it is debubbled together with the ballpoint pen holder while, when debubbled for a long time, an advantageous result in terms of bubbling is obtained.
The definition for xe2x80x9ca long timexe2x80x9d will be as follows. Thus, since ink follower is made of substance which is neither evaporated nor oxidized even when exposed to air at least for 2-3 years in the ballpoint pen holder, centrifugation may be theoretically applied for 2-3 years but, practically, the term xe2x80x9ca long timexe2x80x9d is from one hour to around one day. That is because application of a centrifugal force for a longer time than that will be meaningless and, in addition, separation of oil is resulted due to sedimentation of the thickener itself.